One of the factors which affects the commercial success of mass produced battery powered equipment, such as car radios and portable tape recorders, is the output. The output depends, apart from the electrical characteristics of the equipment, on the input. For example, in the case of a multichannel audio amplifier for a car radio, given the number of channels, the conformation of the final stages and the impedance of the speakers, the only way to increase the output is to increase the input. The input is, on the other hand, determined, in the case of the car radio, by the power of the battery of the vehicle and, in the case of portable equipment, by the number and the capacity of the batteries, which in turn is determined on the basis of the acceptable weight and bulk.
DC/DC converters capable of converting incoming direct current into a higher level of outgoing direct current (step-up DC-DC converter) are commonly used in order to increase output. One application of a converter of this type on an amplifier is shown in FIG. 1. A direct current power supply 10, for example a car battery having a current of 14 V between the positive and negative terminals when the vehicle generator is working, is fitted to a voltage step-up circuit 8, including a DC/DC converter 11 capable of supplying 18 V direct current. Note that, in the case of a car radio system, the change from 14 V to 18 V is advantageous since it is possible to obtain a significant increase in output (around 60%) using audio amplifiers built in an integrated circuit using standard technology. If power above 20 V is required it would be necessary to use special integrated technologies for the amplifiers with considerably higher production costs.
The output of the converter 11, which is also the output of the step-up circuit, is applied to an audio frequency amplifier system 12, for example of the 4 channel type in which each channel includes two coupled 30 W amplifiers. A circuit, which senses the output with a voltage divider Rf1 and Rf2 and stabilizes it, is included in the step-up circuit 8.
The distribution of the power in a system of the type described above, with a 14 V battery and a converter 11 sized in order to allow output of 18 V, is shown in FIG. 2. As can be seen, in order to have an output of 60 W per channel it is necessary to have a DC/DC converter 11 of 300 W, i.e. a large circuit with components capable of operating at high power levels and therefore relatively expensive. As has been shown, the power dissipated--Pdiss--by a converter of this kind is typically around 30 W. The power dissipated by the amplifier 12 has the same value.